Electrostatic shield allowing substantially complete electromagnetic propagation from a transmitter

ABSTRACT

A nickel mesh shield comprised of small closed loops surrounds a transmitter and radiating loop to provide an electrostatic shield for the transmitter. The mesh shield provides substantially no electromagnetic radiation attenuation of the radio frequency transmitted by the transmitter.

[1 1 3,745,% 1 July 10,1973

United States Patent 1 7/1969 Grady,

3,452,597 3,310,736 3/1967 Bayly et al. 1,811,357 6/1931 [75] lnventor:Alan D. Pisano, Revere, Mass. Primary Examiner Roben L AssistantExaminer-Marc E. Bookbinder Attorney-William C. Crutcher et a1.

Assignee: General Electric Company,

Schenectady, NY.

Jan. 20, 1972 [21] Appl. No.: 219,246

[22] Filed:

[52] US. 325/119, 310/68 C, 325/105,

A nickel mesh shield comprised of small closed loops ds a transmitterand radiating loop to provide trostatic shield for the transmitter. Themesh shield provides substantially no electromagnetic radiationattenuation of theradio frequency transmitted by L m 6 k m w 1 mn e S8 m3 I/IOOO/ l 4 O ,9 ,7 4 5 7 3 0 6 1 1 OH 64 5 .1 68 l "0 y H7CQ 2 "55 3mfl 6 9 9 3 O 4 m 3 I n 1 I u 8 5 x m W 9 3 m m0 m 002 E N M 69 1 m own2" 134 3 .u unmz nn Q M 3 am IF B 3 1] 7 8 55 [l 5 Drawing FiguresReferences Cited UNITED STATES PATENTS l0 Claims,

ELECTROSTATIC SHIELD ALLOWING SUBSTANTIALLY COMPLETE ELECTROMAGNETICPROPAGATION FROM A TRANSMITTER BACKGROUND OF THE INVENTION The presentinvention relates to new and improved shielding utilized in theprotection of transmitters. More particularly, the invention relates toa new and improved closed mesh, which shields a transmitter fromelectrostatic gradients incurred during the use of the transmitter forremote reading temperature sensing, yet providing substantially noelectromagnetic or radio frequency radiation attenuation from thetransmitter at the transmitter broadcasting frequency.

The present invention may be utilized advantageously to protect a tunneldiode transmitter such as is disclosed in U. S. Pat. No. 3,260,116,issued to R. F. Grady, Jr., and assigned to the assignee of the presentinvention. The Grady patent referred to herein utilizes a miniaturizedtunnel diode oscillator having a temperature dependent frequency ofoscillation disposed in contact with a current carrying conductor, suchas an armature bar or the like subjected to high potential testing ofthe armature bar or the like.

Since the tunnel diode is a low-impedance, lowcurrent device, itsprotection from possible damage due to high discharge currents to groundin a highly charged and ionized environment, such as incurred duringhigh potential testing of armature bars or the like, has been a problem.

Prior art methods of shielding proved inadequate in that, althoughproviding effective electrostatic shielding, the prior art shieldingwould develop excess heat due to eddy currents and would produce hotspots on the armature bars, which could effect the reliability of thetunnel-diode transmitter. The prior art shields would also shield theelectromagnetic radiation propagation of the radio frequency transmittedby the transmitter, which is the very signal which must be sensed.

A prior art attempt to solve this problem utilized a Faraday shieldwhich eliminated closed loops and thus eliminated eddy currents.However, the Faraday shield was found to be impractical for any smallscale application in that Faraday shields are composed of many hairlikestrands of wire which are formed so that none of them touch each other.The Faraday shield is most difficult to install.

These problems are overcome by my invention, which provides for use of afine closed nickel mesh shield of predetermined thickness andpredetermined hole geometry to provide electrostatic protection to thetunnel diode transmitter and which shield does not interfere with theelectromagnetic radiation propagation of the radio frequency energy fromthe transmitter.

SUMMARY OF THE INVENTION It is an object of this invention to provide ashield which will pass the radio frequency radiation of a transmitterand simultaneously shield the transmitter from the electrostaticgradients which may occur during high potential testing of electricalconductors such as armature bars.

It is a further object of this invention to provide an easy-to-install,fine closed loop nickel mesh shield for a transmitter in the environmentof an armature bar subject to high potential testing.

I predetermined hole geometry.

The invention, both as to its organization and principle of operation,together with further objects and advantages thereof, may better beunderstood by reference to the following detailed description of anembodiment of the invention when taken in conjunction with theaccompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged view of a closedsquare loop mesh shield in accordance with this invention.

FIG. 2 is a sectional view of FIG. 1 showing the thickness of the meshshield in accordance with this invention. w

FIG. 3 is a diagrammatic illustration of the assembling of a transmitterinto an armature bar as utilized with this invention.

FIG. 4 is a diagrammatic illustration of a transmitter encased in anarmature bar protected by a closed loop mesh shield formed in accordancewith this invention.

FIG. 5 is a sectional view of FIG. 4, taken along line VV in accordancewith this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is an enlarged view of aclosed square loop shield mesh 12 formed of a conductive material suchas nickel in accordance with the present invention. Nickel has anextremely low resistivity, on the order of 10" ohm-cm and is a goodelectrostatic shield. When the nickel mesh is formed in accordance withthis invention, negligible radiation attenuation is offered to a timevariant magnetic field ranging from a frequency lower than 60 Hz. toa'frequency higher than 1 MHz.

The desirable electromagnetic radiation properties achieved by thisinvention depend on the type of material, its thickness and the holegeometry of mesh 12.

I First as to thickness, it is well known in the art that an effectivemeans of controlling electromagnetic radi ation at radio frequencies isthrough shields of good conductors, such as copper or aluminum. Magneticflux penetrates such a shield only with great difficulty because as theflux cuts into the conducting material it produces eddy currents thatoppose the penetration. Therefore, if the thickness is a number of timesthe skin depth, relatively all of the radio frequency will beattenuated. Conversely, I find that if ashield is so constructed to havea smaller thickness than the skin depth of the material being used, thiswill be a factor, but not the only factor, in minimum attenuation ofradio frequency radiation by the shield.

Skin depth for nickel may be calculated by utilizing the formula:

E= 1.98 /R/ ,f), inches where R is the resistivity for nickel taken as X10 ohms-cm u, l, where u, relative permeability f frequency in MHz.

Utilizing the foregoing formula, the skin depth as calculated for nickelat a frequency of 1 MHz. is approximately 6.22 mils. When calculated for10 MHz., the skin depth for nickel is approximately 2.0 mils.

It is in accordance with my invention that I choose to utilize nickelmesh having a convenient thickness of 0.5 mils, a thickness which, whileless than the skin depth of nickel, would still undesirably attenuatethe electromagnetic radiation without adequately shielding fromelectrostatic gradients if not for factors mentioned below.

An added factor which gives the desired magnetic shieldingcharacteristics to mesh 12 is an additional physical parameter of thehole geometry. The hole geometry is the ratio of the hole area to thetotal area. The hole geometry is approximately 62,500 to 1,000,000 holesper square inch in a square array and provides a transmissioncharacteristic of slightly greater than 50 percent. This hole geometrycauses any eddy currents produced by the electromagnetic radiation as itpasses mesh 12 to travel in very small loops in mesh 12. The current inadjacent loops travels in the same direction, but in the metal spacesbetween loops the current travels in opposing directions tending tocancel the magnetic effect and results in substantially no loss ofenergy. The effective resistance of a shield to eddy currents caused byelectromagnetic radiation propagation is thus increased greatly overprior art shieldings. Because of the hole geometry utilized in thisinvention, the thickness of the shield may be greater than that of theskin depth if a more rigid mechanical structure is desired. Therefore,the magntic properties of mesh 12 utilized in practicing the inventiondepend on the type of material, its thickness, and the hole geometry.The thickness and hole geometry may be calculated according to formulashereinbefore set forth.

FIG. 2 shows a sectional view of mesh 12 of FIG. 1 to illustrate thatthe thickness of mesh 12 is less than the skin depth E in accordancewith this invention.

Mesh 12 may be formed by being stamped, as shown in FIG. 2, or formed ina woven pattern as is well known in the art. Also, the wire strandswhich form mesh 12 are shown as being circular; however, anyconventional shape will do.

FIGS. 3, 4 and 5 show the application of this invention in theenvironment of thermal testing of armature bars in a dynamoelectricmachine. The embodiments in the following figures in no way limit thescope of the invention but are cited as merely demonstrating a specificapplication.

FIG. 3 shows a transmitter 13 proximate to a copper armature bar 14, thetemperature whereof is to be monitored. For proper installation, a space15, substantially dimensioned to accept a transmitter 13, is formed inthe top of bar 14.

A small area 16 of insulation previously deposited on the top of bar 14is scraped off near the end of space 15. A Nichrome strip 17 or itsequivalent is fastened at area 16 with a suitable solder. This soldermakes an electrical connection between the Nichrome strip 17 and thecopper armature bar 14.

The transmitter 13 is then placed in the space 15. A radiating loop 18is sandwiched between two pieces of epoxy-impregnated glass cloth or thelike and fastened to the bar 14 on the same side as the Nichrome strip16. The loop 18 is positioned so that it is not directly undertransmitter 13 to allow an insulating material to flow down the side ofthe bar 14 and not ruin the loop 18. The Nichrome strip 17 is bent tolie adjacent to and out of contact respective the loop 18.

FIG. 4 shows a nickel mesh 21, formed in accordance with the invention,covering both the transmitter under Nichrome strip 22 and loop 18disposed on bar 14. A

material such as an epoxy-impregnated glass cloth or the like is placedover the mesh 21 to hold the mesh 21 in place and protect it duringinsulation. The bar 14 is then insulated as usual and installed in adynamoelectric machine in a manner well known in the art.

FIG. 5 shows a more detailed view of the mesh 21 taken through line VVin FIG. 4. After the armature bar 14 has cooled, the Nichrome strip 17is bent over the top of armature bar 14 with a small Nichrome strip 22to form a sandwich around nickel mesh 21. The two pieces of Nichrome 17and 22 which surround the mesh 21 are spot-welded together so that theresulting sandwich lies on top of bar 14.

By providing a closed loop mesh of nickel formed in accordane with myinvention and having a predetermined thickness and a predetermined holegeometry around a transmitter mounted respective an armature bar, itwill provide a reliable electrostatic shield without disturbing theelectromagnetic radiation from a transmitter.

While a specific application and embodiment of the invention has beenshown and described, it will be apparent to those skilled in the artthat many more m0di-. fications are possible without departing from theinventive concept herein described. The invention, therefore, is not tobe restricted except as is necessary by the prior art and by the spiritof the appended claims.

What is claimed is:

1. In a dynamoelectric machine having a winding subject to electrostaticradiation during high potential testing wherein a transmitter is to beutilized for remote reading temperature sensing, the combination of:

a transmitter in proximity to said winding and including a radiatingloop;

a mesh formed of a conductive material, comprising a plurality of closedloops shielding said transmitter;

said mesh having a predetermined thickness and a predetermined holegeometry providing an attenuation of the electrostatic radiationproduced by the high potential testing toward said transmitter shieldedby said mesh, and passing substantially complete electromagneticradiation propagation from said transmitter outward of said shieldedtransmitter.

2. The combination as in claim 1 wherein said transmitter is a tunneldiode transmitter.

3. The combination as in claim 1 wherein said thickness of said mesh isless than the skin depth of said conductive material of said mesh.

4. The combination as in claim 1 wherein said closed loops form squares.

mil.

9. The combination as in claim 8 wherein said hole geometry results in aratio of hole area to total area in the order of magnitude of 50percent.

10. The combination as in claim 9 wherein said winding is an armaturebar mounted in said dynamoelectric machine, and wherein said transmitteris disposed in a heat-sensing relationship with said armature bar.

1. In a dynamoelectric machine having a winding subject to electrostaticradiation during high potential testing wherein a transmitter is to beutilized for remote reading temperature sensing, the combination of: atransmitter in proximity to said winding and including a radiating loop;a mesh formed of a conductive material, comprising a plurality of closedloops shielding said transmitter; said mesh having a predeterminedthickness and a predetermined hole geometry providing an attenuation ofthe electrostatic radiation produced by the high potential testingtoward said transmitter shielded by said mesh, and passing substantiallycomplete electromagnetic radiation propagation from said transmitteroutward of said shielded transmitter.
 2. The combination as in claim 1wherein said transmitter is a tunnel diode transmitter.
 3. Thecombination as in claim 1 wherein said thickness of said mesh is lessthan the skin depth of said conductive material of said mesh.
 4. Thecombination as in claim 1 wherein said closed loops form squares.
 5. Thecombination as in claim 1 wherein said hole geometry comprises on theorder of 62,500 holes per square inch in a regular array.
 6. Thecombination as in claim 1 wherein said transmitter broadcastingfrequency is 1 MHz.
 7. The combination as in claim 1 wherein saidconductive material is nickel.
 8. The combination as in claim 7 whereinsaid thickness of said mesh is in the order of magnitude of 0.5 mil. 9.The combination as in claim 8 wherein said hole geometry results in aratio of hole area to total area in the order of magnitude of 50percent.
 10. The combination as in claim 9 wherein said winding is anarmature bar mounted in said dynamoelectric machine, and wherein saidtransmitter is disposed in a heat-sensing relationship with saidarmature bar.